As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery exhibiting high energy density and operating voltage and excellent charge retention and service-life characteristics, which has been widely used as an energy source for various electronic products as well as mobile devices.
Based on their external and internal structures, secondary batteries are generally classified into a cylindrical battery, a prismatic battery, and a pouch-shaped battery. Especially, the prismatic battery and the pouch-shaped battery, which can be stacked with high integration and have a small width to length ratio, have attracted considerable attention.
In addition, the secondary batteries have attracted considerable attention as an energy source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel. As a result, the secondary batteries are being applied to an increasing number of applications owing to advantages thereof, and, in the future, the secondary batteries are expected to be applied to even more applications and products.
As applications and products, to which the secondary batteries are applicable, are increased, kinds of batteries are also increased such that the batteries can provide outputs and capacities corresponding to the various applications and products. In addition, there is a strong need to reduce the size and weight of the batteries applied to the corresponding applications and products.
For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDA), digital cameras, and laptop computers, use one or several small-sized, lightweight battery cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, middle or large-sized devices, such as electric bicycles and hybrid electric vehicles, use a battery module (which may also be referred to as a “middle or large-sized battery pack”) having a plurality of battery cells electrically connected with each other because high output and large capacity are necessary for the middle or large-sized devices. The size and weight of the battery module is directly related to an accommodation space and power of the corresponding middle or large-sized device. For this reason, manufacturers are trying to manufacture small-sized, lightweight battery modules.
Meanwhile, in a conventional battery module, a secondary battery may be abnormally operated due to overcharge, overdischarge, overheating, external impact, etc. with the result that gas may be generated in the battery. For example, the overheated battery generates gas, and decomposition of battery components is further accelerated as the gas is pressurized in a battery case, which causes continuous overheating and generation of gas with the result that a swelling phenomenon may occur. This phenomenon also occurs when the secondary battery is slowly deteriorated due to use of the secondary battery for a long time. In a case in which the battery module is configured to have a structure in which the battery module directly contacts the outer surface of a battery pack, the outer surface of the battery pack may be deformed due to swelling.
In order to solve the above problem, an additional structure is further fixed to the outermost side of the conventional battery module to minimize swelling. In this case, however, the overall volume of a battery module assembly is increased, and a production process is complicated.
Therefore, there is a high necessity for technology that is capable of fundamentally solving the above problems.